As disclosed in PCT International Patent Publication No. WO95/32442, there have been conventionally employed, as multicore plastic optical fibers for communication, a bare multicore plastic optical fiber comprising a plurality of cores of a transparent core resin having a high refractive index and a cladding arranged so as to surround and bundle the cores, a bare multicore plastic optical fiber prepared by coating each core fiber with a cladding resin to form a cladding layer and surrounding the cladded core fibers with a third resin to bundle them, and a multicore plastic optical fiber cable formed by coating the bare multicore optical fibers with a sheathing resin.
Single core plastic optical fibers formed by coating a core with a cladding resin in two layers are disclosed in Japanese Patent Application Laid-Open Nos. 204209/1987, 51206/1992 and 249325/1993. However, the fibers disclosed therein are single core plastic optical fibers, not multicore plastic optical fibers. Therefore, they have a large core diameter and insufficient light retention upon bending. In case of the conventional multicore plastic optical fibers, when the numerical aperture of the fibers (hereinafter referred to as Fiber NA) is decreased in order to widen signal transmission band width, and when the numerical aperture of incident light source (hereinafter referred to as LNA) is larger than Fiber NA, the fibers cannot receive a light beyond the Fiber NA. Accordingly, the conventional multicore plastic optical fibers have a drawback that the light requirement is small. Another drawback of the conventional multicore plastic optical fibers is light loss caused by bending the fibers. Usually, a multicore plastic optical fiber can reduce light loss caused by bending since each core diameter of the multicore plastic optical fiber can be made very small. Nevertheless, when Fiber NA becomes small, the light loss upon bending unpreferably becomes too large to ignore. Even in the case that Fiber NA is relatively large, a plastic optical fiber having smaller light loss upon bending is more preferable.
An object of the present invention is to provide such a plastic optical fiber with desirable transmission bandwidth that receives a larger quantity of light from a light source and causes less light loss upon bending the fiber.